I like your idea of experimenting if you just wanted to fly it would be
cheeper to just buy a redy to fly kit.

I put some comments in between below;

"sebastian" wrote.
Hi Jan I really appreciate the feedback,

Paragliders are inherently very very slow. A typical paramotor will
have between 15-30hp a very moderate climb rate of 150-300ft/min,
climb speed of 15-20mph, and cruise at about 25-30mph, max speed
~35mph on good day. Part of the reason im setting out to design my
own is that i plan to make use of an electric motor rather that
internal combustion engine. Im using a dc motor that produces maxium
power (15hp) at 48v with 72 rpm/volt =3456rpm. But i may try to run
it at 36v first =2592rpm. One of the nice things about this motor is [How
many HP is it developing at 36V?]
its increadibly high and flat torque across the rpm range.

so constant speed propeller= no twist but changes angle of attack [
Constant speed propellers keep the RPM at the from cockpit set RPM, The
propeller still have a twist ]
and constant pitch propeller =constant change in angle over length [
Correct]
(twist) how is that measured... angle change/inch??? [ Normaly the pitch
is measured at different Stations along the blade, like 15, 30, 45, 60, 75,
and 90% of the radii.]
fixed angle propeller =no change in pitch or angle of attack [Called Fixed
Pitch Propellers.]
[ A constantspeed/adjusteble prop can have a constant PITCH at a certain
angle measured at the 75% radii (like15 deg.) from tip to about 40% radii.
The blade angle change ofcourse with the radii to keep the pitch. When the
Pitch/angle is changed (blade rotated) the pitch will no longer be constant,
with higher then the 15 deg setting the PITCH will be greater at the tip
then at the 40% radii ]

how do i do i measure or determine loading... or since i know my
torque will be good at any speed should i design the propeller for
maximum power which i know is going to be ~3400rpm. [You want the engine
to turn at maximum at max speed, or with a less powerful engine near max rpm
at climb speed, then you will cruise at near max RPM but with reduced power,
that is good with en electric engine because it will use less energy at high
RPM then at reduced RPM with more propeller load. ]

and how do i factor number of blades in the the equation...simpler
would be better but wont more blades allow more thrust per rpm and
smaller diameter and lower tip speed/noise...part of the reason im
going electric is to make it as quiet as possible... [ I don't think the
tip speed will be a problem, a 2 blad prop will be better with its larger
diameter, what you want/need is max thrust with the extra weight from
batteries and the relativ weak engine. With the 15 HP/3450 RPM you will end
up with a 43"- 45" diameter, 38,5" with 3 blade, and 35,5" with 4 blade!]

Jan Carlsson
www.jcpropellerdesign.com



thanks!!!!



'Jan Carlsson" wrote in message
...
Sebastian,

With data on both engine and speed it is easy to calculate an
propeller.

I sugest you make a wood propeller first, then you can see how much prop
you
need, The Idea is to "load" the engine so you get the desired RPM, with
the
most optimum prop you can get at the design "point" (speed)
Even if it is calculated correctly there can be unknown factors, like if
the
engine turn out 2 HP more or less!?
Then it is easy quick and cheep to make a new wood propeller

The blade have to be twisted, you have to know the differens of Pitch
and
Angle, an prop with uniform pitch along the blade have the blade twisted
to
higher angle closer to the hub. Even with reduced pitch near the hub,
the
angle is normaly higher there.

With the Prop behind the back(pack) It will work in turbulant air,
specially
near the hub it will be "dead" air, closer to the tip it will work in
the
"aircrafts" forward speed + the induced air (air sucked in)
The Induced airspeed will be large compered to forward speed.

What is the normal Climb speed, Cruise speed and top speed for a
paramotor?

Jan Carlsson
www.jcpropellerdesign.com

"sebastian" skrev i meddelandet
om...
then youre saying the best way to answer the questions
is...empirically.
or can i make decisions a priori about prop geometry based on my input
power (15hp) and rpm (2500)? it would seem that long non twisting,
tapered, symetric airfoil blades that are ground adjustable in pitch
and varied in the nr of blades on the hub would be the better way
since the the variables of pitch, diameter(shortening), & nr of blades
could be experimented with and optimized. I suppose under these
circumstances all effort should be put into aspect ratio since that is
the one varibale that would be most difficult to vary and test. so
with that said, any advice on choosing an aspect ratio for my purpose?
and as a i vary these parameters...how do i measure changes in thrust
directly...attach my motor to a scale somehow?

"Jan Carlsson" wrote in message
...
A constant pitch will be better then a constant angle and blade
width.

The Q is what Pitch, what Diameter, what Aspect Ratio, what nr of
blades?

The questions is the same whatever it is a one meter model plane,
back
pak
or a turbo-prop.

Jan Carlsson


"sebastian" skrev i meddelandet
om...
ah well, that was exactly the type of feeback i was looking for
thanks.

My revised plan is to experiment with building a carbon fiber
propeller. I like the general design theory behind the wind
turbine
that i specified, so I am thinking of inverting the pitch for
thrust
and making the blade shorter and broader (45-50" diameter, 2-4"
wide
tapering to tip). My plan is to make foam airfoil cores with a
hotwire cutter, extensively reinforce the lengthwise direction
with
unidirectional carbon fiber roving, and wrap the whole blade with
carbon fiber farbric and vacuum bag. I would use a similar
twisting
geometry that narrows and goes from ~20 degrees to nearly a flat
pitch
at the tip and transforms from an elipitcal cross section at the
hub
to an airfoil at the tip. or would it be better and simpler to
have a
constant pitch, width, and airfoil and make the overall pitch
ground
adjustable. Rememeber i need it to be as light and efficent as
possible this is for a paraglider fan (backpack type) not some big
beefy airplane. Any suggestions?